1. Field of the Invention
The present invention relates to a valve spring device attached to an engine of, for example, an automobile and a valve train using the valve spring device.
2. Description of the Related Art
A valve train of an internal combustion engine (hereinafter referred to simply as the engine) includes an intake valve and an exhaust valve attached to a cylinder head, a valve spring, a cam mechanism, etc. The intake and exhaust valves serve individually to open and close ports in the cylinder head. The valve mechanism urges these valves in their closing direction. An installation load and a maximum load of the valve spring are set to suitable values such that the valves never undergo a jump or bounce when the engine is in high rotation. The jump is a phenomenon that a valve that is opened or closed at a high speed by a cam hops up above a predetermined valve opening position near the maximum lift of the valve. The bounce is a phenomenon that the valve is repelled by a valve seat and fails to maintain a fully closed state when it is closed as the cam rotates.
It is generally known that the valve spring causes surging if it is driven at a high speed when the engine is in high rotation. If the surging occurs, the load of the valve spring fluctuates. In some cases, the surging may cause the valve spring to deform in a direction to compress itself. A reduction of the load that is attributable to this phenomenon may bring about the jump or bounce. Thus, the maximum load of the valve spring is set to a relatively large value in consideration of the surging. The larger the load of the valve spring, however, the greater a friction loss of a valve train system is, so that the fuel efficiency of the engine may be worsened. Thus, the load of the valve spring is expected to be reduced.
A double-pitch coil spring has been proposed as a means to prevent an increase of surging. The double-pitch coil spring includes a small-pitch portion with small wire turn pitches and a large-pitch portion with large pitches. If any surging occurs, turns of the small-pitch portion closely contact one another, thereby restraining the surging to some extent.
In this double-pitch coil spring, however, both the small- and large-pitch portions may possibly vibrate to reduce the load, depending on the resonance frequency, when the surging increases. In such a case, the jump or bounce sometimes cannot be suppressed. In the double-pitch coil spring, moreover, a twisting vibration that is caused by the surging propagates from the large-pitch portion to the small-pitch portion. Thus, the double-pitch coil spring has a problem that its surging restraining effect is not large enough for a large surging, in particular.
An example of a valve spring device for surging prevention is described in Jpn. Pat. Appln. KOKAI Publication No. 2000-240705. This valve spring device is provided with a valve spring, formed of a compression coil spring, and an elastically deformable washer. The washer is located in series with the valve spring. A surging wave that is produced in the valve spring is absorbed by elastic deformation of the washer.
In the valve spring device described in the above patent document, the washer is compressed as the valve spring is compressed. Specifically, the washer starts to deform at an initial stage of a valve lift, and a deflection of the washer increases with an increase of that of the valve spring. If the valve spring undergoes any surging, therefore, the washer deforms in the direction of its compression. Thus, a force that presses the valve is reduced to cause a jump or bounce of the valve. If the spring constant of the valve spring is increased in order to enhance the force to press the valve, however, a friction loss of a valve train system increases, so that the fuel efficiency of an engine worsens.